The problem of bringing sunlight to illuminate the interior of a building is one of the oldest and most difficult in the history of architecture. The atrium plan, in which a building is constructed around a central open courtyard, was basic in ancient Roman architecture. It consisted of a central opening in the roof ("compluvium") which allowed light into the center of the building. That type of structure, as well as atriums with glass roofs, is still widely used.
Another solution to bringing sunlight indoors is windows, which originally were holes in walls. With the inexpensive manufacture of sheet glass, both vertical windows and skylights (horizontal windows) have become the most common means of bringing sunlight into a building.
However, it often happens that windows do not provide enough natural light, for example, in basements or rooms whose windows are blocked or shaded. Lamps may be needed for reading and general illumination even in daylight hours. In addition, artificial light, generally electric light using special bulbs or phosphors, may approximate the spectral characteristics of natural light. Such lamps are used in the treatment of SAD (Seasonal Affect Disorder) but are not widely used for indoor lighting. The lack of sunlight has been attributed as a cause of seasonal depression, especially in northern countries.
Fiber optics consist of an elongated transparent strand having a core and a cladding, each having a different index of refraction. Light is conducted along its length. Fiber optics is used to conduct light for communications and surgical instruments. It has also been used with a bright bulb, i.e., 400 watt halogen bulb, to bring light to illuminate swimming pools and other environments in which it is difficult to mount an electric light fixture.
U.S. Pat. No. 4,306,769, entitled "Interior Illumination Apparatus Using Sunlight", describes a collector, mounted on a roof, a light pipe and a diffuser. The collector is larger than a cross-section of a hollow light pipe tube having reflective internal walls. However, this system requires a large opening in the roof and a large area for the light pipe. The light pipe is not flexible. A number of these "light pipes" are commercially available from Solatube (TM), Sun Tunnel (TM) and Vista EZ Light (TM) and are promoted as an inexpensive alternative to a skylight. They generally consist of rigid aluminum tubes that fit together; but the Sun Tunnel is a flexible tube somewhat like an oversized clothes-dryer vent.
A number of patents are directed to light guides. For example, U.S. Pat. No. 5,799,124 uses a bulb as its light source and a liquid-filled flexible plastic tube to conduct the light. The light leaks from the light pipe to illuminate a swimming pool.
U.S. Pat. No. 4,957,347 is assigned to Lumenyte International, Costa Mesa, Calif., a manufacturer of illumination plastic optic fiber. That patent describes an optical conduit which emits light along its length. The polymer core, of high refractive index, is surrounded by a heat-shrunk cladding of Teflon (TM of DuPont), i.e., fluorinated ethylene propylene having a low refractive index. Other companies commercially selling fiber optic light guides include Fiberstairs, Fiberoptic Lighting, Prime Light and TPR Enterprises.